Automated control valves such as, for example, cage guided control valves, are often used in process control plants or systems to control the flow of process fluids. A cage-guided control valve typically includes an actuator (e.g., a pneumatic actuator, an electric actuator, a hydraulic actuator, etc.) operatively coupled to a first end of a valve shaft that extends along a longitudinal axis. Typically, the actuator includes an actuator housing that comprises an upper actuator casing and a lower actuator casing, and a diaphragm is secured between the upper and lower actuator casings of the actuator housing. A diaphragm plate is disposed below the diaphragm within an interior volume of the lower actuator casing, and a first end of the valve shaft is coupled to a bottom portion of the diaphragm plate. When pressurized fluid is introduced into the interior volume of the lower actuator casing and/or an interior volume of the upper actuator casing, the diaphragm plate displaces. The displacement of the diaphragm plate displaces the valve shaft along the longitudinal axis, which in turn displaces a valve plug secured to a second end of the valve shaft, thereby opening and closing the valve. Because the interior volume of the lower actuator casing is adapted to receive pressurized fluid, an air-tight seal must be maintained between the valve stem and the portion of the lower actuator casing that accepts the valve stem. This air-tight seal limits or prevents the valve shaft (and the diaphragm plate) from displacing relative to the actuator housing in a direction normal to the longitudinal axis.
Frequently, only the interior volume of the upper actuator casing receives pressurized fluid to displace the valve shaft along the longitudinal axis. Because the interior volume of the lower actuator casing does not receive pressurized fluid, no air-tight seal is necessary between the valve stem and the portion of the lower actuator casing that accepts the valve stem. In this case, the valve stem and the diaphragm plate may transversely displace relative to the actuator housing. Such transverse displacement of the valve stem and the diaphragm plate relative to the actuator housing may damage any of all of the parts due to mutual contact. For example, such contact may create friction and/or may cause the diaphragm plate to crimp or deform the diaphragm. This transverse displacement may also result in positioner failures, packing failures, and other general performance issues.
To prevent such transverse displacement, especially in valves that are used in seismic service, such as nuclear applications, an upper guide may be disposed within the interior volume of the upper actuator casing to stabilize the valve shaft and the diaphragm plate relative to the actuator housing. However, such an upper guide requires a specially casted diaphragm plate and/or an upper actuator housing, a modified valve stem, as well as the use of a bushing and additional seals. Moreover, the upper guide adds weight and changes the center of gravity of the actuator assembly, both of which are undesirable in seismic environments.